I'd like to add to the thread my own headcanon about Apep, who could have been Lilith. Lilith could assume the form of a serpent (as in Revelations of the Dark Mother she was the one to tempt Eve in the Garden by assuming the form of the serpent). The serpent is actually one of her sacred animals in lore. Darkness is also under her purview, as she inherited domain over the night from Lucifer Morningstar. Apep's attempt to devour the sun could be a mytholgical version of Lilith's quarrel with G-d.

The implication that Set / Sutekh could have slayed Lilith I couldn't totally reconcile...